Switching circuits have been widely utilized in DC-DC converters, inverters, or the like. There are many possibilities that typical switching circuits utilize transistors such as MOSFETs, IGBTs, or the like. In switching circuits, since voltages applied to gates of transistors are switched, signal paths between major electrodes (namely, sources and drains in case of unipolar transistors such as MOSFET etc., collectors and emitters in case of bipolar transistors such as IGBT etc.) of the transistors are switched in a temporal manner between a conducting state and a non-conducting state.
Various sorts of requirements have been made with respect to switching circuits, namely, these switching circuits are required to be made compact, and are required to be arranged by employing a small number of circuit components; ON-resistances of the switching circuits must be low, and switching losses thereof must be low; and surge voltages of the switching circuits must be suppressed to low surge voltages.
In order to make the switching circuits compact, there is a merit if frequencies at which gate voltages are changed are increased. This reason is given that since the frequencies are made high, inductances and capacitances required by the switching circuits can be suppressed to small values.
If operating frequencies of switching circuits are increased, then switching losses of transistors may cause serious problems. In order to suppress these switching losses to small values, there is a merit if switching speeds of the transistors are increased. Since researching/developing operations of these transistors are progressed, the switching speeds thereof are increased.
However, even in the present stage, it is practically difficult to suppress both the switching losses and the surge voltages at the same time. In order to suppress the switching losses to small values, it is advantage to increase the switching speeds of the transistors. However, if the switching speeds are increased, then the surge voltages are increased. In order to suppress the surge voltages to low voltages, it is advantage to decrease the switching speeds of the transistors. However, if the switching speeds are decreased, then the switching losses are increased. A trade-off relationship is present between the switching losses and the surge voltages. If the switching losses are suppressed to the small values, then the surge voltages are increased, whereas if the surge voltages are suppressed to the low voltages, then the switching losses are increased. It is practically difficult that both the switching losses and the surge voltages are suppressed to the low values at the same time.
In order to suppress both a surge voltage and a switching loss to a low surge voltage and a low switching loss, such a gate driving circuit for switching gate resistors in a stepwise manner in connection with a time elapse has been proposed. However, this technical idea has problems. That is, the gate driving circuit becomes complex, and a total number of electronic components required to manufacture a switching circuit is necessarily increased.
Under such a circumstance, the transistor described in JP-A-2004-6598 (corresponding to U.S. Pat. No. 6,700,156) has been developed. This transistor is a MOSFET in which a p layer containing a p type impurity in low concentration has been formed at a position located adjusted to a p base layer.
In the case of a normal MOSFET, the higher a voltage between the source and the drain of the normal MOSFET becomes, the lower a capacitance between the drain and the gate thereof is decreased. When the capacitance between the drain and the gate is small, a voltage change speed between the drain and the source becomes high, which is generated when the transistor is turned OFF. In the normal MOSFET, the higher the voltage between the drain and the source is increased when the MOSFET is turned OFF, the higher the voltage change speed between the drain and the source becomes, so that a surge voltage is increased.
As described in JP-A-2004-6598, if the p layer containing the p type impurity in the low concentration is added to the position located adjacent to the p base layer, then the following characteristic can be obtained. That is, the higher the voltage between the drain and the source becomes, the larger the capacitance between the gate and the drain is increased. When this MOSFET is employed, if the transistor is turned OFF so that the voltage between the drain and the source is increased, then the capacitance between the gate and the drain is increased. As a result, the voltage change speed between the drain and the source is decreased, so that the surge voltage can be suppressed to the low surge voltage.
However, in the above-described MOSFET, the below-mentioned problem occurs. That is, the current flows through the p layer containing the p type impurity in the low concentration, so that the ON resistance thereof is increased. Also, the stronger the surge voltage is suppressed, the higher the ON resistance of the transistor is increased. Furthermore, in order to effectively suppress the surge voltage, the p layer containing the p type impurity in the low concentration must be made large. If the p layer is made large, then the cell pitch of the transistor becomes large, so that the ON resistance of the transistor is increased. In other words, in accordance with the measure manner for improving the internal structure of the transistor, it is practically difficult to suppress the ON resistance, the surge voltage, and the switching loss to the lower values.
As a consequence, it is desirable to accomplish switching circuits capable of suppressing ON resistances to lower ON resistance values, capable of suppressing switching losses to lower switching losses, and also capable of suppressing surge voltages to lower surge voltages, while the switching circuits can be constructed by employing small numbers of structural components.
Also, other switching circuits are known in the field. In the switching circuits, power supplies and loads are series-connected between one pair of major electrodes of transistors; and since the transistors are turned ON/OFF, conditions under which electric power is supplied to the loads, and conditions under which the electric power is not supplied are selectively switched. For instance, while an inverter circuit contains a switching circuit, a transistor of the switching circuit is turned ON/OFF in order that DC power is converted into AC power, and then, the converted AC power is supplied to a motor (one example of load).
One example as to this sort of switching circuits has been disclosed in JP-A-6-326579. The switching circuit has been equipped with a series circuit connected between a gate electrode and a drain electrode of a transistor, in which a zener diode has been series-connected to a diode. The zener diode has been set in such a manner that when a surge voltage is generated on a wiring line on the drain electrode side of the transistor, the zener diode may break down in response to the generated surge voltage. The function of the diode is to avoid that a gate current flowing to the gate electrode of the transistor when the transistor is turned ON flows via the series circuit to the wiring line on the drain electrode side of the transistor.
In the above-described switching circuit, when such a surge voltage is generated on the wiring line on the drain electrode side of the transistor, the zener diode breaks down. This surge voltage is higher than a total voltage (VZD+VF) of a breakdown voltage “VZD” of the zener voltage and a forward direction voltage “VF” of the diode. When the zener diode breaks down, the current flows from the wiring line of the drain electrode side of the transistor via the series circuit to the gate electrode of the transistor, so that the voltage of the gate electrode is increased, and thus, the transistor is turned ON. This switching circuit can be operated in such a manner that when the surge voltage is generated, since the transistor is instantaneously turned ON, the surge energy may be discharged via the transistor to the external circuit.
However, the breakdown voltage VZD of the zener diode contains fluctuations of approximately of ±10% due to manufacturing tolerance. As a consequence, in this switching circuit, depending upon zener diodes to be employed, timing when these zener diodes break down is fluctuated. As a result, the above-described switching circuit has such a problem that the timing at which the surge voltage is discharged outside the own switching circuit cannot be set in higher precision.
Another type of switching circuit is desirable. That is, not only timing for slowing a changing speed of a voltage at a control electrode of a transistor is made stable, but also the changing speed of the voltage at the control electrode of the transistor is slowed at earlier timing than such a timing when a voltage at a major electrode of the transistor on the high voltage side exceeds a power supply voltage, so that the suppression capability for the surge voltage can be improved.
Also, in a switching circuit equipped with a transistor (MOSFET), an adverse influence caused by a surge voltage in the case that the MOSFET is brought into a turn-OFF state is required to be reduced.
As indicated in FIG. 34, a load driving circuit capable of protecting a MOSFET 5 from a surge voltage by the circuit itself is disclosed in JP-A-6-326579. A diode 9a and a zener diode 9 have been series-connected to a branch line 310 under such a condition that the diode 9a is located opposite to the zener diode 9. The diode 9a plays a role of suppressing that a current flows through the branch line 310 when the MOSFET 5 is turned ON in response to an ON signal outputted from an output terminal 318 of a control circuit 3. The zener diode 9 plays a role of increasing a voltage at a gate G of the MOSFET 5 in such a case that a positive surge voltage is applied to a power supply line 322 when the MOSFET 5 is turned OFF. The positive surge voltage is higher than, or equal to the zener voltage “VZD” plus the diode forward direction threshold voltage “VF.” As a result, the MOSFET 5 is brought into an ON state, so that a signal path between the source and the drain of the MOSFET 5 becomes conductive, and thus, the surge voltage can be discharged.
However, normally, a zener voltage “VZD” of a zener diode is fluctuated by approximately ±10%. Accordingly, in such a case that the zener diode is employed as explained in the above-described conventional technique, it is practically difficult that the reference voltage (VZD+VF) for suppressing the serge voltage is correctly set to a predetermined value.
FIG. 35 represents a result obtained by simulating a temporal change of a drain voltage VD when a MOSFET is turned OFF in such a case that a voltage of a power supply PS is set to 100V. FIG. 35 shows a simulation result in the case that no surge voltage suppression measure is performed in the switching circuit (indicated by line “XXXVA”); a simulation result in the case that the serve voltage suppression measure using the zener diode 9 is performed as explained in the above-described conventional load driving circuit, and then, “VZD+VF” is set to 100 V (represented by line “XXXVB”); and a simulation result in the case that although the serve voltage suppression measure using the zener diode 9 is performed as explained in the above-described conventional load driving circuit, “VZD+VF” is varied to 90 V and 110 V due to fluctuations of the zener diode “VZD” (indicated by lines “XXXVC” and “XXXVD”).
Assuming now that a surge voltage is defined as a difference from a peak value of the drain voltage VD up to a value under steady condition, when the surge voltage suppression measure is not taken, the surge voltage is 22V; and when “VZD+VF” is set to 100 V, the surge voltage is 3 V, so that the suppression effect of the surge voltage may become achieved. However, if (VZD+VF) becomes 110 V, then the surge voltage becomes 13 V, so that the suppression effect of the surge voltage is considerably lowered. Also, when (VZD+VF) becomes 90 V, the drain voltage VD cannot be recovered to 100 V corresponding to the power supply voltage.
As previously described, in the surge voltage suppressing circuits using the zener diodes, when VZD+VF can be correctly controlled to the predetermined value, the surge voltage suppressing effect can be achieved. However, as explained above, since the zener voltage VZD is fluctuated, the suppression effect of the surge voltage may not be sufficiently achieved, or the drain voltage VD may not be recovered up to the power supply voltage.
As a consequence, such a switching circuit capable of suppressing a surge voltage under stable condition is required, and another switching circuit capable of reducing both a switching loss and noise of a transistor is required.
Also, another switching circuit is known in the field. In the switching circuit, since a transistor connected to a load is turned ON/OFF, a condition under which electric power is supplied to the load, and another condition under which the electric power is not supplied are selectively switched. For instance, in an inverter circuit, a transistor is turned ON/OFF in order that DC power is converted into AC power, and then, the converted AC power is supplied to a motor. The turn-ON/OFF operations of the transistor of this sort of circuit are controlled by a driving circuit connected to the gate electrode (otherwise, base electrode) of this transistor.
FIG. 65A to FIG. 65E show an example of operating waveform diagrams as the related art in such a case that a field-effect type transistor is employed as this sort of transistor. A driving circuit switches ON/OFF operations of the field-effect transistor by applying a driving voltage “Vin” to a gate electrode of the field-effect transistor.
A first description is made of a transition time period during which the field-effect transistor is turned ON. When the driving voltage Vin becomes a high level from a low level, a positive gate current “Ig” flows toward the gate electrode of the transistor, so that electron charges are stored in the gate electrode. When the electron charges are stored in the gate electrode, a gate-to-source voltage “Vgs” of the transistor is increased. When the gate-to-source voltage “Vgs” of the transistor is increased, a drain current “Id” starts to flow from the drain of the transistor to the source thereof, so that a drain-to-source voltage Vds is decreased. The turn-OFF operation of the transistor is transferred to the turn-ON operation via these operation stages.
Next, a description is made of a transition time period T100 during which the field-effect transistor is turned OFF. When the driving voltage Vin becomes a low level from a high level, the electron charges stored in the gate electrode are discharged, and a negative gate current “Ig” flows from the gate electrode toward the driving circuit, so that the gate-to-source voltage “Vgs” of the transistor is decreased.
When the gate-to-source voltage “Vgs” of the transistor is decreased, the drain current “Id” is also decreased, so that the drain-to-source voltage Vds is increased. The turn-ON operation of the transistor is transferred to the turn-OFF operation via these operation stages.
As shown in FIG. 65A to FIG. 65E, in a final stage of the transition time period T100 during which the transistor is turned OFF, a surge voltage is generated in the drain-to-source voltage Vds. This surge voltage is induced by the drain current Id which is steeply varied, and an inductance staying on the wiring line of the drain electrode side within the circuit.
In order to suppress the increase of this surge voltage, the drain current Id may be gently varied. For instance, if the gate resistance of the transistor is increased, then the speed at which the electron charges stored in the gate electrode are discharged is decreased, so that the negative gate current Ig flows in a gentle manner. As a result, the drain current Id is also gently decreased, so that increasing of the surge voltage can be suppressed. However, if the drain current Id of the transistor is gently decreased, then a time period required until the transistor is turned OFF is increased, so that a turn-OFF loss is increased. That is to say, this sort of transistors have a trade-off relationship between the surge voltage and the turn-OFF loss during the transition time period T100 of the turn-OFF operation of the transistor.
In order to overcome this trade-off relationship, it is desirable to steeply vary the drain current Id in the beginning stage of the transition time period T100 of the turn-OFF operation, and also, it is desirable to gently vary the drain current Id in the final stage of the transition time period T100 of the turn-OFF operation. If the drain current Id is steeply varied in the beginning stage of the transition time period T100, then the time required for turning OFF the transistor can be shortened. As a result, the turn-OFF loss can be suppressed to a small loss. If the drain current Id is gently varied in the final stage of the transition time period T100, then increasing of the surge voltage can be suppressed.
JP-A-6-291631 discloses such a technical idea capable of adjusting a resistance value of a gate resistor of a transistor based upon voltages between major electrodes of the transistor. As the voltages between the major electrodes, there are a voltage between a drain electrode and a source electrode, a voltage between a collector electrode and an emitter electrode, and so on. In this technical idea, the following adjustment is carried out: That is, when the voltage between the major electrodes of the transistor is high, the resistance value of the gate is increased, whereas when the voltage between the major electrodes of the transistor is low, the resistance value of the gate is decreased. Concretely speaking, this driving circuit has been equipped with a resistance variable means connected to the gate electrode of the transistor. The resistance variable means has been arranged by a semiconductor switching element and a fixed resistor which is parallel-connected to the semiconductor switching element. When the voltage between the major electrodes of the transistor is higher than a predetermined value, the semiconductor switching element is turned OFF, whereas when the voltage between the major electrodes of the transistor is lower than the predetermined value, the semiconductor switching element is turned ON. In other words, when the voltage between the major electrodes of the transistor is high, the semiconductor switching element is turned OFF, so that the gate resistance is adjusted to be increased in response to a resistance value of a fixed resistor. When the voltage between the major electrodes of the transistor is low, the semiconductor switching element is turned ON, so that the gate resistance is adjusted to be decreased in response to an internal resistance value of the semiconductor switching element.
When the above-described driving circuit is utilized, in the beginning stage (when voltage between major electrodes is low) of the transition time period for the turning-OFF operation of the semiconductor switching element, the semiconductor switching element is turned ON, so that the resistance value of the gate resistor is adjusted to become small, and thus, the gate current is steeply varied. As a result, the drain current of the transistor is steeply varied, so that the time required for turning OFF the semiconductor switching element can be shortened. Furthermore, in the final stage (when voltage between major electrodes is high) of the transition time period for the turning-OFF operation, the semiconductor switching element is turned OFF, so that the resistance value of the gate resistor is adjusted to become large, and thus, the gate current is gently varied. As a result, the drain current of the transistor is gently varied, so that increasing of surge voltage can be suppressed.
As a consequence, in the above-described driving circuit, the high resistance state of the gate resistor is realized by utilizing the fixed resistor having the high resistance value. In order to suppress increasing of the surge voltage, it is desirable that the resistance value of the fixed resistor is set to be large. However, the fixed resistor having the higher resistance value may increase the turn-OFF loss. As a consequence, in order to suppress increasing of the turn-OFF loss, such a timing when the resistor is switched to the fixed resistor having the high resistance value by turning ON/OFF the semiconductor switching element must be correctly set to the final stage of the transition time period for turning ON/OFF the semiconductor switch element. In the final stage of the transition time period for turning ON/OFF the semiconductor switching element, the voltage between the major electrodes of the transistor has been reached to the higher voltage state. In the above-described driving circuit, the turn ON/OFF operations of the semiconductor switching element must be controlled by correctly changing the voltage between the major electrodes of this transistor up to the threshold value as to the turn-ON/OFF operations of the semiconductor switching element. As a consequence, in order to realize such a circuit, a total number of necessary circuit components is increased, so that cost of this circuit is necessarily increased.
Another technical idea is required which may adjust the resistance value of the gate resistor of the transistor based upon the voltage between the major electrodes of the transistor in accordance with another manner different from the above-described manner. It should also be noted that the problems have been described by mainly considering the transition time period for turning OFF the transistor in the above-mentioned descriptions. However, even in a transition time period for turning ON the transistor, there are many possibilities that the resistance value of the gate resistor of the transistor is wanted to be adjusted based upon the voltage between the major electrodes of the transistor. In other words, such a technical idea capable of achieving useful results is required even in any transition time periods for turning ON and turning OFF the transistor.
Also, JP-A-1-183214 discloses a circuit for diving a bipolar type transistor. It should also be noted that the technical idea related to this driving circuit may also be utilized in such a case that a field-effect type transistor is driven.
The above-described driving circuit has been equipped with two resistors connected to a gate electrode of a unipolar type transistor. In accordance with this driving circuit, in a beginning stage of a transition time period for turning OFF the transistor, a negative gate current from the gate electrode flows through the two resistors. On the other hand, in a final stage of the transition time period for turning OFF the transistor, a negative gate current from the gate electrode flows only through the other resistor, while one resistor is cut off.
If the above-described driving circuit is utilized, then the negative gate current is steeply varied in the beginning stage of the transition time period for turning OFF the transistor, so that the drain current is steeply varied, and thus, the time required for turning OFF the transistor can be shortened. Moreover, if the above-described driving circuit is utilized, then the negative gate current is gently varied in the final stage of the transition time period for turning OFF the transistor, so that the drain current is gently varied, and thus, increasing of the surge voltage can be suppressed.
In the above-described driving circuit, timing for cutting off one resistor has been previously set based upon a time constant as to both a capacitor and a resistor assembled in the driving circuit. As a consequence, when the turn-OFF operation is repeatedly carried out, such an event may occur. That is, the timing for cutting off one resistor is deviated from such a timing for determining both the beginning stage and the final stage of the turn-OFF operation. The manner for controlling the cut-off timing which has been previously set cannot be synchronized with the operation of the transistor. As a consequence, increasing of the surge voltage and increasing of the turn-OFF loss cannot be firmly suppressed.
As a consequence, the below-mentioned technical idea is desired. That is, while monitoring such a condition that a transistor is operated, a resistance value of a gate resistor of the transistor is adjusted. It should also be understood that the problems have been described by mainly considering the transition time period for turning OFF the transistor in the above-mentioned descriptions. However, even in a transition time period for turning ON the transistor, there are many possibilities that the below-mentioned technical idea is required. That is, while monitoring such a condition that the transistor is operated, the resistance value of the gate resistor of the transistor is adjusted, even in the transition time period for turning ON the transistor. As a consequence, such a technical idea capable of achieving useful results is required even in any of the transition time periods for turning OFF and turning ON the transistor.